The invention relates to an adhesion-promoting system for rubber goods.
Good adhesion of vulcanised rubber to steel cord and particularly, brass-plated steel cord, as well as synthetical fibre cord made from polyester, rayon, polyamides, aramide fibres, carbon fibres and glass fibres is of decisive importance for the long-term properties of e.g., textile and of steel radial tyres. Most frequently used adhesion promoters are based on so-called resorcinol novolaks (EP 0 440 036 A1) or resorcinol itself (U.S. Pat. No. 4,148,769 A; DE 20 02 023 A1; B2 DE 24 14 798 B2). Pure resorcinol is used only in combination with methylene or formaldehyde donors, such as hexamethylene tetramine (“hexa”) or hexamethoxymethyl melamine (HMMM). The resins condensed in this manner contribute to the reinforcing effect and improve the interfacial adhesion while at the same time providing increased rigidity of the rubber blend and improved wear resistance of the tread under a wide range of stress conditions. However, the use of resorcinol alone presents health and environmental protection problems because resorcinol may evaporate under the rubber processing conditions. In contrast to free resorcinol, resorcinol-formaldehyde condensates (resorcinol novolaks) release substantially less vapours and are used in combination with HMMM, although they are less effective for the adhesion than free resorcinol. At the same time, the stress values and especially the Shore A hardness of reinforced rubber blends modified with such resorcinol novolaks are substantially less than those achieved when free resorcinol is used. Furthermore, owing to the strong natural colour, resorcinol/formaldehyde condensates cannot be used in those rubber blends intended for applications in which a pale colour of the vulcanised rubber is required. Moreover, resorcinol based or resorcinol containing adhesion promoters show premature aging particularly under heat and/or higher humidity.
Resorcinol-formaldehyde condensates can be obtained, for example, by condensation of formaldehyde or formaldehyde donors with resorcinol or mixtures of resorcinol and a further phenol.
Resorcinol-free adhesion promoters which are based on functionalised melamine resins (EP 0 473 948 A2) and which have a good adhesion promoting action similar to that of free resorcinol or resorcinol novolaks are also available. However, rubber blends which contain such functionalised melamine resins vulcanise more slowly than those which contain HMMM/resorcinol systems. Resorcinol-free systems based on resins prepared from phenol, alkylcarbamates, and formaldehyde have been described in EP 0 827 971 B1. In these systems, resorcinol is replaced by modified novolaks which are prepared by simultaneous reaction of a polyhydric phenol with an aldehyde and an alkyl urethane in the presence of an acidic catalyst at elevated temperature and contain no interfering amounts of free starting phenol. From the Polish patent application PL 136 592, a carbamate-modified phenolic resin has been known that has a good adhesion to metals and glass. This resin is prepared by simultaneous reaction of phenol and a carbamate (1-hexylurethane) with formaldehyde. These systems have the drawback that residual unreacted urethane remains in the resin which is at least partially liberated when the resin is incorporated into the rubber mixture. This leads to undesired fuming.